1. Field of the Invention
The present invention relates to a medical diagnostic imaging apparatus, a non-destructive testing apparatus, and a detection apparatus and a detection system for use in an analysis apparatus using radiation or the like.
2. Description of the Related Art
Thin-film semiconductor technology has been used to produce sensing and imaging devices, such as a photosensor apparatus or a radiation detection apparatus using an array of pixels. In this type of apparatuses, each pixel includes a combination of a switch element such as a TFT (thin film transistor) and a conversion element such as a photoelectric conversion element. Particularly, in recent years, to meet a requirement for higher sensitivity and higher operating speed, an effort has been made to achieve the requirement by producing a pixel array of a detection apparatus by using polycrystalline semiconductor TFTs as switching elements. In general, the pixel structure used in a detection apparatus can be classified into two types, i.e., a single-plane type in which a conversion element and a switch element are disposed in the same plane and a stacked type in which a conversion element is disposed above (stacked upon) a switch element. In the production of single-plane type pixels, the conversion element and the switch element can be produced using the same semiconductor production process, which allows simplification of the production process. On the other hand, in the production of stacked-type pixels, the provision of the conversion element above the switch element makes it possible to increase the size of the conversion element in each pixel compared with the single-plane type, and thus an increased aperture ratio can be obtained. The aperture ratio is generally understood as the ratio between the transparent area of a pixel (excluding the pixel's wiring area) and the whole pixel area. In the single-plane type, the area occupied by the switch element and wiring reduces the aperture ratio. In contrast, in the stacked type, because the switch element is stacked with the conversion element and because microfabrication enables reduction in the size of the wiring, the aspect ratio is increased. Therefore, in a stacked-type detection apparatus, it is possible to increase optical efficiency (the amount of radiation or light incident on the active area of the pixel) compared with the single-plane type detection apparatus. This makes it possible to provide a detection apparatus with a high signal-to-noise (S/N) ratio and high optical sensitivity.
Regarding such a detection apparatus, Japanese Patent Application Laid-Open No. 2008-085029 discloses a photoelectric conversion apparatus, using a pixel including a photoelectric conversion element disposed above a switch element on a substrate, configured such that capacitance of the pixel is increased to increase the light intensity (the amount of radiation) acceptable by the pixel. More specifically, in the photoelectric conversion apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-085029, a capacitor is directly connected to a photoelectric conversion element and the capacitor is disposed in a region where the capacitor overlaps the photoelectric conversion element when seen in plan view. In the above-described apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-085029, because the capacitor is directly connected to the photoelectric conversion element, the pixel has a fixed capacitance value which cannot be adjusted.
In a detection apparatus used to take a radiographic image for medical diagnosis, there is a need for a detection apparatus capable of capturing both still images and moving images. In capturing radiographic images, the amount of radiation used in taking one moving image is about 1/100 of that used in capturing one still image. Therefore, there is a large difference in the maximum amount of radiation acceptable by the pixel between moving and still images. For example, if a moving image is captured using a pixel with large capacitance set to be optimum for detecting high radiation density in capturing still images, the capacitance of the pixel may be too large in capturing moving images, and thus there is a possibility that a resultant signal is not sufficiently high. On the other hand, in a case where a still image is captured using a pixel with small capacitance set to be optimum for detecting low radiation density in capturing moving images, the capacitance of the pixel may be too small for capturing still images, and thus there is a possibility that saturation occurs in the pixel, and a resultant signal does not include necessary image information.
U.S. Patent Application Publication No. 2002/0190215 discloses a detection apparatus configured such that a capacitor is connected to a conversion element via a switch to achieve a capability of controlling the capacitance of the conversion element by controlling the switch.